"Current information suggests that al-Qaida and affiliated organizations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the State Department said in a statement.

The State Department alerted US citizens to the "continued potential for terrorist attacks, particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula".

The statement said the worldwide travel alert would expire on August 31. According to the alert, terrorists may select to use a variety of means and weapons and target both official and private interests.

US citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure. "Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services. US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling," it said.

Noting that the US continues to work closely with other nations on the threat from international terrorism, including from al-Qaida, the State Department said information is routinely shared between the US and our key partners in order to disrupt terrorist plotting, identify and take action against potential operatives, and strengthen US defences against potential threats.

A day earlier, the State Department had announced the closure of certain embassies and consulates in countries where its missions are open on Sunday, in particular August 4. "The Department of State has instructed certain US embassies and consulates to remain closed or to suspend operations on Sunday, August 4th," State Department spokesperson Marie Harf told reporters at her daily news conference.